1. Field of the Invention
The present invention is directed to a computed tomography apparatus of the type having a detector array whose output signals are processed into shadowgrams in a computer, for which purpose a relative displacement ensues between the measuring unit composed of an x-ray source and detector array, and the patient support in the longitudinal direction of the support, and the output signals are acquired by the computer at a predetermined angular position of the measuring unit.
2. Description of the Prior Art
In a computed tomography apparatus of the third generation, it is known to rotate the measuring unit around the system axis by 360xc2x0 for the presentation of a specific slice of the examination subject and to calculate the tomogram from the output signals of the detector array. For defining the slice that is to be tomographically presented, the measuring unit is locked against rotation in a predetermined angular position, and a relative displacement between the measuring unit and the patient support ensues proceeding from a start position. The length for this relative displacement is preset. Subsequently, radiation is triggered and the output signals of the detector array are acquired by the computer during the scanning. The shadowgram is calculated from the intermediately stored and partially processed output signals of the detector array only after the preset relative displacement, that corresponds to the desired length of the shadowgram has been traversed. The calculation of the shadowgram, therefore ensues only after the patient support has reached the preset final position.
Presetting the length of the shadowgram means an additional operating procedure. Since the presentation of the shadowgram ensues only after the measurement, it is possible that a measurement that is either too short or too long can occur. In the former instance, the registration of the shadowgram must be repeated; the patient is exposed to unnecessary radiation in both instances.
It is also known to fashion a computed tomography apparatus of the third or fourth generation such that a shadowgram can be produced simultaneously with the exposure (German OS 41 03 588). 
Moreover, German PS 42 23 430 discloses a computed tomography apparatus with a detector array whose output signals are also processed to shadowgrams in a computer in addition to being processed into tomograms, with a relative displacement ensuing between the measuring unit and the patient support in the longitudinal direction of said support, and the output signals are acquired by the computer at predetermined angular positions of the measuring unit. A convolution of the output signals in longitudinal direction (Z-direction) of the support ensues according to the following equation:             b      l        ⁡          (      k      )        =            ∑              K        =                              -            n                    /          2                            n        /        2              ⁢          xe2x80x83        ⁢                            U          l                ⁡                  (                      k            -            K                    )                    *              h        ⁡                  (          K          )                    
wherein
* means xe2x80x9cconvolved withxe2x80x9d
l the index in the xcex2-direction
k the index in the Z-direction
xcexa the count index for the convolution
Ul(k) the readings convolved in the xcex2-direction (measurement of the attenuation values of a slice)
Ul(kr) the most recently registered reading
bl(k) the readings additionally convolved in the Z-direction
h the convolution kernel
n the degree of the convolution kernel
xcex2 the longitudinal direction of the detector array.
An object of the present invention is to provide a teaching for a computed tomography apparatus with improved image quality in the production of shadowgrams.
This object is inventively achieved in a computed tomography apparatus wherein the shadowgram is already reproduced on a monitor during the relative displacement between the patient support and the measuring unit, the shadowgram being reproduced according to the progress of this relative displacement. The measuring system can be locked in a predetermined angular position, so that only a shadowgram can be produced in the measurement. However, the measuring system also can rotate around the examination subject during the exposure, so that a shadowgram and a tomogram are simultaneously generated. The physician can end the measurement when the desired field of measurement is recognized in the shadowgram.